Anthropometrics/Body Segment Parameters

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Segment Mass • Segment mass is a quantitative indicator of a segment’s inertia • Determine by segment density and volume • For many segments density can be estimated from literature data, and volume can be measured Segment Mass • Tables also exist that allow one to predict segment masses, usually as a percentage of whole body mass • Finally, the volume represented by different tissues (fat, muscle, bone) can also be determined using modern imaging/scanning techniques (MRI, CT, DEXA) • These volumes can be combined with more accurate individual tissue densities to yield good estimates of segment mass Center of Mass (COM) • Body segments are made up of an immense number of distributed mass particles • Mechanical analysis is facilitated by identifying the COM: a single point about which the mass is equally distributed (i.e., the balance point) • Typically specified as a distance (% of segment length) relative to some unique point (e.g. proximal joint center) Segmental COM The location of the COM can be defined as follows: x 1 m 1 m 2 m 3 x 2 x 3 X M M X = Σ m i x i so X = Σ m i x i M Segmental COM The COM of an object can be determined using a “reaction board” COM is found by knowing that all moments about the knife edge must sum to zero Segmental COM cog = X X cog = Y Y prox prox + L%(X + L%(Y dist dist - X - Y prox prox ) ) The x and y coordinates for the right shoulder of a female tennis player are . The x and y coordinates for the elbow are . What is the location of the center of gravity of the upper arm? should be at knife edge 2
Whole-body COM Whole-body COM Σ(w r) = (Σw) r com Σ(w r) = (ΣW) r com A bar (length = 2.2 m; mass = 20 kg) is loaded with two 20-kg plates on its right side (locked in place, 35 and 40 cm from the right end) and two 20-kg plates on its left side (locked in place, 30 and 20 cm from the left end). Where is the COM of this bar and the four plates, relative to the left end of the bar? chapter05_files/wholebodycom.xls Moment of Inertia (I) I is the sum of the products of the (1) mass element of an object and (2) square of the distance between the mass element and axis of rotation axis I AXIS = Σm i r i 2 r 1 m 1 m 2 r 2 Radius of Gyration A more practical approach: I AXIS = (m BODY )(k 2 ) k indicates radius of gyration, which is an experimentally determined length that applies to the whole object at once k depends on the location of the axis and location of the mass I AXIS = m 1 r 12 + m 2 r 22 + m 3 r 32 + .... + m n r n 2 Radius of Gyration Mass Moment of Inertia • Radius of gyration is the distance a particle with the same mass as the segment would need to be from the axis of rotation to have the same moment of inertia (angular inertia) as the segment • Radius of gyration is really just a convenient way of “packaging” mass moment of inertia information • In anthropometric data sets, k is often expressed relative to segment length For a three dimensional object there will be 3 moments of inertia, typically expressed about the 3 principle axes of the segment For human limb segments, I X and I Z will be similar, I Y will be much smaller Z SEG X SEG Y SEG 3
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Anthropometrics/Body Segment Parameters

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